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Abstract:

Unabsorbed infrared laser light that has passed though plastic parts to
be welded with a low absorption TTIr process is recirculated in an
infinite loop back to the low absorption weld interface for reabsorption
in the process. On each pass in the infinite loop, more infrared laser
light is absorbed at the weld interface.

Claims:

1. A method for welding first and second plastic parts in a low
absorption through-transmission infrared welding process, the method
comprising; directing a beam of infrared laser light from a source of
infrared laser light to the parts where it impinges the first part,
transits through the first part to a weld interface at a junction of the
first and second part where there is low absorptivity to infrared laser
light at the weld interface, and a portion of the infrared laser light
passes through the second part and exits the second part, the infrared
laser light exiting the second part being unabsorbed infrared laser
light; and redirecting the unabsorbed infrared laser light back to the
weld interface in an infinite loop where the redirected infrared laser
light impinges and transits through the first part, is partially absorbed
at the weld interface, and a portion of the redirected infrared laser
light not absorbed continues through the second part and exits the far
side of the second part wherein in each pass of the infinite loop, more
infrared laser light is absorbed at the weld interface.

2. The method of claim 1 including redirecting the unabsorbed infrared
laser light with a fiber optic.

3. The method of claim 1 including redirecting the unabsorbed infrared
laser right with a waveguide.

4. The method of claim 1 wherein the parts are tubular parts with the
first part coaxially surrounding the second part, and redirecting the
unabsorbed infrared laser light includes redirecting the unabsorbed laser
light with a cylindrical mirror that coaxially surrounds the tubular
parts.

5. The method of claim 4 wherein redirecting the unabsorbed infrared
laser light with the cylindrical mirror includes redirecting it so that
the unabsorbed infrared laser light eventually impinges the weld
interface from all directions.

6. The method of claim 1 wherein the parts are tubular parts including a
fitting and redirecting the unabsorbed infrared laser light includes
redirecting the unabsorbed laser light with a spherical mirror that
surrounds the parts.

7. The method of claim 6 wherein redirecting the unabsorbed infrared
laser light with the spherical mirror includes redirecting it so that the
unabsorbed infrared laser light eventually impinges the weld interface
from all directions.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.
No. 12/114,847 filed on May 5, 2008 which claims the benefit of U.S.
Provisional Application No. 60/927,898 for Photon Recirculator For
Plastics Welding And Method Of Plastic Welding filed on May 4, 2007. The
entire disclosures of the above applications are incorporated herein by
reference.

FIELD

[0002] The present disclosure relates to plastic welding, and more
particularly to laser welding of plastic parts.

BACKGROUND

[0003] Laser welding is commonly used to join plastic or resinous parts,
such as thermoplastic parts, at a welding zone. An example of such use of
lasers can be found in U.S. Pat. No. 4,636,609, which is expressly
incorporated herein by reference.

[0004] As is well known, lasers provide a semi-focused beam of
electromagnetic radiation at a specified frequency (i.e., coherent
monochromatic radiation). There are a number of types of lasers
available; however, infrared lasers or non-coherent sources provide a
relatively economical source of radiative energy for use in heating a
welding zone. One particular example of infrared welding is known as
Through-Transmission Infrared (TTIr) Welding. TTIr welding employs an
infrared laser capable of producing infrared radiation that is directed
by lenses, diffractive optics, fiber optics, waveguides, lightpipes, or
lightguides through a first plastic part and into a second plastic part.
This first plastic part is often referred to as the transmissive piece,
since it generally permits the laser beam from the laser to pass
therethrough. However, the second plastic part is often referred to as
absorptive piece, since this piece (and/or an absorptive additive at the
weld interface) generally absorbs the radiative energy of the laser beam
to produce heat in the welding zone. This heat in the welding zone causes
the transmissive piece and the absorptive piece to be melted and, with
intimate contact, welded together.

[0005] With reference to FIGS. 1A and 1B, typical through transmission
infrared (TTIr) systems 100 and 100' for laser welding of plastics are
shown. A beam of infrared laser light 102 from a source of infrared laser
light 104 is directed to the plastic parts 106, 108 to be welded. The
infrared laser light passes through transmissive plastic part 106 to a
weld interface 110 at a junction of transmissive plastic part and
absorptive plastic part 108. Weld interface 110 is also sometimes
referred to in the art as a weld site, a weld region or a weld area. An
infrared absorber additive 112 may be provided at weld interface 110
(FIG. 1A) The absorption of the laser light heats up the weld interface
at the junction of the parts 106, 108, melting the plastic in both parts
106, 108 at the weld interface 110. The laser light is removed, such as
by turning laser source 102 off, after an appropriate period of time and
the molten plastic at weld interface 110 then cools, thus welding the two
plastic parts 106, 108 together.

[0006] Oftentimes, the absorptive second plastic part 108, or the infrared
absorber additive 112 used at the weld interface 110, are relatively low
absorbers of the infrared light. A large portion, indicated at 114, of
the infrared laser light 102 then passes though both parts 106, 108 and
out of part 108, becoming wasted in the process.

[0007] With low absorbers, either too low a laser energy is delivered to
the weld interface 110 to make a weld, or relatively high laser energies
need to be used to translate into enough energy at the weld interface 110
to make a weld.

SUMMARY

[0008] In accordance with an aspect of the present disclosure, unabsorbed
infrared laser light that has passed though plastic parts to be welded
with a low absorption TTIr process is recirculated back to the low
absorption weld interface for reabsorption in the process. A beam of
infrared laser light is directed at first and second plastic parts to be
welded. The infrared laser light impinges and transits through the first
part to a weld interface at the junction of the parts. At the weld
interface, at which there is low absorbtivity to the infrared laser
light, the infrared laser light is partially absorbed by an additive
infrared absorber; the infrared laser light is partially absorbed by the
second part, or both. The portion of the infrared laser light that is not
absorbed continues through the second part and exits a far side of the
second part. The unabsorbed infrared laser light exiting the second part
is then redirected back to the weld interface in an infinite loop where
the redirected infrared laser light impinges and transits through the
first part, is partially absorbed at the weld interface, and a portion of
the redirected infrared laser light not absorbed continues through the
second part and exits the far side of the second part. In each loop,
additional infrared laser light is absorbed at the weld interface.

[0009] The first part may be a transmissive part and the second part may
have low absorbtivity to infrared laser light.

[0010] In an aspect, the parts are tubular parts with the first part
coaxially surrounding the second part. The infrared laser light is
redirected with a cylindrical mirror that coaxially surrounds the tubular
parts. In an aspect, the cylindrical mirror redirects the infrared laser
light so that the infrared laser light eventually impinges the tubular
parts from all directions.

[0011] In an aspect, the parts are tubular parts including a fitting and
redirecting the unabsorbed infrared laser light includes redirecting the
unabsorbed laser light with a spherical mirror that surrounds the parts.
In an aspect, redirecting the unabsorbed infrared laser light with the
spherical mirror includes redirecting it so that the unabsorbed infrared
laser light eventually impinges the weld interface from all directions.

[0012] Further areas of applicability will become apparent from the
description provided herein. It should be understood that the description
and specific examples are intended for purposes of illustration only and
are not intended to limit the scope of the present disclosure.

DRAWINGS

[0013] The drawings described herein are for illustration purposes only
and are not intended to limit the scope of the present disclosure in any
way.

[0015] FIG. 2 is a schematic illustration of a low absorption TTIr laser
welding system having an infinite loop for redirection of infrared laser
light through a weld interface between the parts to be welded in
accordance with an aspect of the present disclosure;

[0016] FIG. 3 is a schematic illustration of a low absorption TTIr laser
welding system having a an infinite loop for multiple path redirection of
infrared laser light through a weld interface in accordance with an
aspect of the present disclosure;

[0018] FIG. 5 is a schematic illustration of a low absorption TTIr laser
welding system welding tubular parts in which unabsorbed infrared laser
light is recirculated to the tubular parts; and

[0019] FIG. 6 is a schematic illustration of a low absorption TTIr laser
welding system welding tubular parts in which one of the tubular parts is
a fitting in which unabsorbed infrared laser light is recirculated to the
parts.

DETAILED DESCRIPTION

[0020] The following description is merely exemplary in nature and is not
intended to limit the present disclosure, application, or uses. It should
be understood that throughout the drawings, corresponding reference
numerals indicate like or corresponding parts and features.

[0021] In accordance with an aspect of the present disclosure, unabsorbed
infrared laser light that has passed though plastic parts to be welded
with a low absorption TTIr process is recirculated back to the low
absorption weld interface for reabsorption in the process. A beam of
infrared laser light is directed at the plastic parts to be welded, a
transmissive first part and an absorptive (or partially absorptive)
second part. The infrared laser light impinges the transmissive part and
first transits through the transmissive part to be welded to the weld
interface at the junction of the two parts. At the weld interface, either
the infrared laser light is partially absorbed by an additive infrared
absorber, the infrared laser light is partially absorbed by the
absorptive part, or both.

[0022] The portion of the infrared laser light that is not absorbed
continues through the absorptive part and exits the far side. This
infrared laser light is then redirected back to the weld interface either
by using mirrors, waveguides, or fiber optics. On the second pass, more
infrared laser light is absorbed in the partially absorbing medium (the
additive infrared absorber, the absorptive part, or both).

[0023] Again, with the second pass, some infrared laser light will not be
absorbed, and will pass through the parts to be welded. This infrared
laser light again can be redirected back to the weld interface. This
process can be repeated any number of times, and in some variants, can
have an infinite repetition. Even with a low absolute infrared laser
light absorption ratio of the parts on each pass, eventually a high
percentage of infrared laser light will be absorbed at the weld
interface.

[0024] When the infrared laser light is redirected at the parts to be
welded, it can be redirected in the same direction as the original
impinging light, or can be redirected at from some other angle that
points towards the weld region. FIG. 2 shows a laser welding system 200
having a photon recirculator 202 having an infinite loop. In the
illustrative embodiment shown in FIG. 2, infinite photon recirculator
includes a fiber optic loop 204 that extends from where the infrared
laser light exits absorptive part 108 back to where the laser light
originally impinges on transmissive part 106. By redirecting the infrared
laser light in the same direction as the infrared laser light 102
originally impinged on the transmissive part directly from the source 104
of laser light 102, an infinite loop is set up as shown in FIG. 2.

[0025] In a variation, the infrared laser light is redirected to the weld
interface from another angle, that is, redirected to the weld interface
in a direction different than the direction of the original impinging
infrared laser light. In this variation, multiple pass angles are
illustratively set up as shown in FIG. 3 and the path of the infrared
laser light can be redirected by one or more mirrors to provide a photon
recirculator 301 that provides two or more passes of the infrared laser
light 102 through the weld interface 110. In the illustrative embodiment
shown in FIG. 3, laser welding system 300 includes a plurality of
mirrors, such as three mirrors 302, 304, 306. A beam of infrared laser
light 102 is directed to parts 106, 108 to be welded. The infrared laser
light 102 first transits through transmissive part 106 to weld interface
110. The portion of infrared laser light 102 that is not absorbed is
reflected by mirror 302 to mirror 304, by mirror 304 back through parts
106, 108 where it passes through weld interface 110. The unabsorbed
portion of infrared laser light 102 reflected by mirror 304 exiting
transmissive part 106 is reflected by mirror 306 back through parts 106,
108 where it passes through weld interface 110. The unabsorbed portion of
infrared laser light 102 reflected by mirror 306 is reflected by mirror
304 to mirror 302 which reflects it back through parts 106, 108 where it
passes through weld interface 110. In the embodiment of FIG. 3, infrared
laser light 102 makes four passes through parts 106, 108 and weld
interface 110.

[0026] With reference to FIG. 4, for tubular plastic parts, the infrared
laser light 102 impinging on the tubular parts 402, 404 refracts as if
the tubular parts 402, 404 form a lens as the infrared laser light passes
through the parts 402, 404. This fans out the infrared laser light 102 in
an approximate half circle on the opposite side that the infrared laser
light 102 first impinged on outer part 402, as seen in FIG. 4.
Illustratively, part 402 is the transmissive plastic part and coaxially
surrounds part 404, which is the absorptive plastic part.

[0027] FIG. 5 shows a laser welding system 500 having a photon
recirculator 502 that recirculates the fanned out infrared laser light
102 back to the parts 402, 404 to be welded. In the embodiment of FIG. 5,
photon recirculator 502 includes a cylindrical mirror 504 disposed
coaxially around the tubular parts 402, 404. Illustratively, cylindrical
mirror 504 includes an opening in which infrared laser light 102 is
directed by infrared laser light source 104. Cylindrical mirror 504
reflects the fanned out infrared laser light 102 to recirculate the
fanned out infrared laser light back to the tubular parts 402, 404.
Cylindrical mirror 504 has a geometry that continues to recirculate the
laser light so that eventually the infrared laser light impinges from all
directions around the tubular parts 402, 404 to be welded as seen in FIG.
5. Eventually, most of the laser light 102 is absorbed by the low
absorption absorbers used in the welding process, which is the absorptive
plastic part 404, an infrared absorber additive such as infrared absorber
additive 112 (FIG. 1) disposed at a junction of tubular parts 402, 404,
or both.

[0028] It should be understood that cylindrical mirror 504 need not be a
continuous cylinder. For example, cylindrical mirror 504 may include
slots to facilitate the use of a conveyor system to move tubular parts
402, 404 into and out of cylindrical mirror 504.

[0029] FIG. 6 shows a laser welding system 600 for use in welding tubular
parts 602, 604 in which tubular part 604 is a fitting, such as a
junction, elbow, union, or the like. Illustratively, ends of tubular
parts 602 are received in fitting 604. Where surfaces of parts 602, 604
abut each other is the weld interface in this illustrative embodiment.
Fitting 604 may illustratively be the transmissive part and tubular parts
602 the absorptive parts. It should be understood that tubular parts 602
could be the transmissive parts and fitting 604 the absorptive part, in
which case fitting 604 may illustratively be received in ends of fitting
602. It should also be understood that an infrared absorber additive
could be disposed at the weld interface interface(s) between tubular
parts 602 and junction 604.

[0030] Laser welding system 600 includes a photon recirculator 606 that
recirculates laser light 102 that passes through parts 602, 604, which is
fanned out by parts 602, 604, back to the weld interface 110. Photon
recirculator 606 includes a spherical mirror 608. In the embodiment of
FIG. 6, spherical mirror 608 includes opposed first and second half
spherical mirrors 610 spaced from each other by space 612 to facilitate
the placement of parts 602, 604 in photon circulator 606.

[0031] It should be understood that mirrors, waveguides or fiber optics
can be used to redirect the infrared laser light. Mirrors have the
advantage of high efficiency. Waveguides and fiber optics have the
advantage of more flexibility of geometry than the straight lightpaths
needed with mirrors. Waveguides and fiber optics have a greater optical
acceptance angle than a mirror train, which is useful in an infinite loop
arrangement.

[0032] The infrared laser light can be redirected to the weld interface
for any number of passes. A single additional pass, or a low number of
passes has the advantage of simplicity. A large number of passes has the
advantage of greater total absorption efficiency by the parts to be
welded.

[0033] With tubular parts, the coaxial cylindrical mirror arrangement
advantageously directs the infrared laser light from all angles to the
tubular parts to be welded, and sets up an infinite loop of recirculation
which yields a high total absorption of the low absorption absorbers in
the weld process.

[0034] The mirrors can be metallic, have a high reflection efficiency thin
film coating, or be reflective prisms. The waveguides can be either
positive transmissive dielectric waveguides of negative reflective
waveguides. The fiber optics can be single mode fiber, multimode fiber,
selfoc fiber, holey fibers, or hollow fiber.

[0035] The plastic parts to be welded can use an infrared absorbing
additive at the weld interface, or can use a volume infrared absorber in
one (or both) parts. It is assumed, with infrared laser light
recirculating, that the infrared absorbers are not total absorbers, so
that some of the infrared light escapes from the parts to be welded from
the initial pass.

[0036] The infrared laser light used in the process can be an infrared
laser or a broadband infrared source. A collimated infrared laser is more
directable and therefore more applicable with mirrors.

[0037] Recirculating the infrared laser light greatly increases the
welding efficiency, and allows for welding of parts in a low absorption
process, that otherwise could not be welded. Less laser or broadband
infrared laser light power needs to be used, thus lowering the cost of
the welding machine.

[0038] Recirculating infrared laser light with tubular parts both improves
the overall absorption of the process, and decreases the complexity of
optics needed to deliver infrared light from all angles to the tubular
assembly.